1. Field of the Invention
The present invention relates generally to an ink jet head and a method of manufacturing the ink jet head, and more particularly to a technique for forming ink-jet holes of the head.
2. Discussion of the Related Art
An ink jet head using actuator elements for producing an ink jetting energy for each ink-jet hole is known as a so-called "drop-on-demand" type ink jet head. For instance, the actuator element uses a piezoelectric ceramic material which undergoes deformation upon electric energization thereof, to change a volume of an ink chamber filled with an ink, so that a droplet of the ink is jetted through an ink-jet hole communicating with the ink chamber, when the volume of the ink chamber is reduced. When the volume of the ink chamber is increased, a certain amount of the ink is introduced into the ink chamber through an ink inlet. The actuator elements corresponding to the ink chambers are selectively energized according to printing data, so that the ink droplets are jetted from the ink-jet holes corresponding to the energized actuator elements, whereby a desired image such as a character or graphical representation is formed in a matrix of dots in the form of the ink droplets on an appropriate recording medium such as a paper sheet or web placed in opposed relationship with the ink jet head.
An example of this type of ink jet head is disclosed in EP-A-0277703, EP-A-0278589 and EP-A-0278590.
WO91/17051 discloses an ink jet head having a high density of ink-jet holes, in which the ink-jet holes are formed in two parallel rows.
The ink jet head disclosed in EP-A-0277703, EP-A-0278589 and EP-A-0278590 indicated above is shown generally at 1 in FIG. 14. This ink jet head 1 consists of a piezoelectric ceramic plate 2, a cover plate 3, a nozzle plate 31 and a substrate 41.
The piezoelectric ceramic plate 2 is subjected to a polarization treatment in which the plate 2 is polarized in a direction indicated by an arrow 5 in FIG. 16. The polarized plate 2 is then subjected to a machining operation in which a plurality of parallel grooves 8 are formed by suitable cutting tool such as a diamond blade in the form of a disk having a small thickness, which defines the width of each groove 8. The parallel grooves 8 are defined by parallel partition walls 11 which are equally spaced apart from each other in a direction perpendicular to the direction of extension of the grooves 8. The parallel grooves 8 have a constant depth over a predetermined length from a front end face 4 of the plate 2, and terminate into respective shallow grooves 16 formed adjacent to a rear end face 15 of the plate 2. Namely, the depth of the rear end portion of each groove 8 decreases as it approaches the shallow groove 16. A pair of metal electrodes 13 are formed by a suitable film-forming technique such as sputtering, in the form of strips on upper halves of the opposed side surfaces of the adjacent partition walls 11 which define each groove 8. Metal electrodes 9 are formed by sputtering, for example, on the opposed side surfaces of the partition walls 11 which define each shallow groove 16, and also on the bottom surface of each shallow groove 16.
The cover plate 3 is formed of a glass material, a ceramic material, a resin material or any other suitable material. The cover plate 3 has an ink inlet 21 and a manifold 22 formed by a suitable technique such as grinding or machining. The piezoelectric ceramic plate 2 and the cover plate 3 are bonded together by an epoxy resin adhesive or other suitable bonding agent, at their surfaces in which the grooves 8 and manifold 22 are formed. Thus, there is prepared an ink-chamber member generally indicated at 26 in FIG. 16, in which the grooves 8 and shallow grooves 16 are closed at their upper openings by the cover plate 3, whereby a plurality of ink chambers 12 are formed. The ink chambers 12 communicate with the manifold 22, at the shallow grooves 16 whose rear ends are closed by the cover plate 3. As indicated in FIG. 16, the ink chambers 12 are equally spaced apart from each other in the direction perpendicular to the direction of extension thereof. Each ink chamber 12 has a rectangular cross sectional shape and has a relatively large length and a relatively small width. In operation of the ink jet head 1, the ink chambers 12 are filled with an ink introduced through the ink inlet 21 and manifold 22.
The ink-chamber member 26 is bonded to the substrate 41 by a suitable bonding agent such as an epoxy resin, at the surface of the piezoelectric ceramic plate 2 which is opposite to the surface in which the grooves 8, 16 are formed. As shown in FIG. 14, the substrate 41 is provided with conductive strips 42 aligned with the respective ink chambers 12. The conductive strips 42 are electrically connected by conductor wires 43 to the respective electrodes 9 which cover the bottom surfaces of the shallow grooves 16.
To the front end face 4 of the ink-chamber member 26, there is bonded the nozzle plate 31 which has a plurality of ink-jet holes 32 arranged in a row such that the ink-jet holes 32 communicate with the respective ink chambers 12.
The conductive strips 42 formed on the substrate 41 are electrically connected to an LSI chip 51, as shown in FIG. 15. To the LSI chip 5, there are connected a clock line 52, a data line 53, a voltage line 54 and a ground line 55. The LSI chip 5 operates according to clock pulses received from the clock line 52, and is adapted to apply a predetermined voltage V of the voltage line 54 to the appropriate conductive strips 42 in response to drive commands received from the data line 53, so that the ink chambers 12 specified by the drive commands are deformed so as to reduce their volume by application of the voltage V through the corresponding pairs of electrodes 13, whereby ink droplets are delivered from the ink-jet holes 32 corresponding to the deformed ink chambers 12. Thus, the drive commands received from the data line 53 specify the ink-jet holes 32 from which the ink droplets are delivered. The conductive strips 42 and electrodes 13 which correspond to the other ink-jet holes 32 are maintained at the ground voltage (0 V) of the ground line 55.
The ink jet head 1 constructed as described above operates as follows:
When the LSI chip 51 receives a drive command to deliver an ink droplet from the ink-jet hole 32 communicating with the ink chamber 12b (FIG. 16), for example, the predetermined drive voltage V is applied between the opposed electrodes 13e and 13f, while the electrodes 13d and 13g are grounded. As a result, the partition wall 11b partially defining the ink chamber 12b is exposed to an electric field in a direction indicated by arrow 14b in FIG. 17, while the partition wall 11c also partially defining the ink chamber 12b is exposed to an electric field in a direction indicated by arrow 14c in FIG. 17. Since the directions 14b, 14c of the electric fields are normal to the direction of polarization of the piezoelectric ceramic plate 2 indicated by arrow 5, the partition walls 11b, 11c undergo deflection or flexure to to a piezoelectric effect. Consequently, the volume of the ink chamber 12b is reduced due to the deflection of the partition walls 11b, 11c, causing a rapid increase of the pressure of the ink within the ink chamber 12b, whereby the ink is forced to flow from the ink chamber 12b to the ink-jet hole 32 which communicates with the ink chamber 12b. Thus, an ink droplet is jetted from that ink-jet hole 32. When the application of the drive voltage V to the electrodes 13e, 13f is cut off, the partition walls 11b, 11c are restored relatively slowly to their original position, and the pressure of the ink within the ink chamber 12b is lowered at a relatively low rate, whereby a certain amount of the ink is introduced into the ink chamber 12b through the ink inlet 21 and manifold 22.
The operation described above is a basic operation of the conventional ink jet head. However, the ink jet head may be operated in various modes. For instance, a drive voltage is applied to the electrodes so as to increase the volume of the selected ink chamber 12b to thereby introduce a certain amount of the ink into the ink chamber 12b. Then, the drive voltage is removed from the electrodes to restore the ink chamber 12b to its original state (FIG. 16) to thereby deliver a droplet of the ink from the ink-jet hole 32.
The nozzle plate 31 having the ink-jet holes 32 used in the ink jet head of the type described above is conventionally produced by performing a suitable operation such as pressing or drilling on a blank to form the ink-jet holes 32, or by using a high-energy beam such as an excimer laser beam to form the ink-jet holes 32 through a sheet-like blank (31), as disclosed in JP-A-61-32761 as indicated in FIG. 18.
Another method of producing a nozzle plate is disclosed in JP-A-3-297651, wherein the nozzle plate is formed by nickel electrocasting or injection molding. Referring to FIGS. 19 through FIG. 22, there will be described conventional methods of producing a nozzle plate by injection molding. A nozzle plate 71 as shown in FIG. 19 is formed by injection molding using a mold as shown in FIGS. 20(a) and 20(b), while a nozzle plate 81 as shown in FIG. 21 is formed by injection molding using a mold as shown in FIGS. 22(a) and 22(b). The nozzle plate 71 has ink-jet holes each consisting of an orifice portion 72 and a tapered portion 73 which communicates with the ink chamber 12 when the nozzle plate 71 is bonded to the piezoelectric ceramic plate 3. Similarly, the nozzle plate 81 ink-jet holes each consisting of a orifice portion 82 and a tapered portion 83. FIG. 20(b) is a cross sectional view taken along line A--A in FIG. 20(a), while FIG. 22(b) is a cross sectional view taken along line A--A of FIG. 22(a). Reference numerals 170, 180 denote a core used in the mold. The core 170, 180 has a row of projections 173, 183 as shown in FIGS. 20(b) and 22(b), which correspond to a row of ink-jet holes 72, 73, 82, 83 to be formed in the nozzle plate 71, 81. To form the nozzle plate 71, 81, a suitable material is introduced into the injection mold through a gate 100 to fill a mold cavity which is partially defined by the core 170, 180.
EP-A-0309146 shows a method of forming tapered ink-jet holes by applying an excimer laser beam to a nozzle plate blank while the angle of the axis of the beam relative to the blank is changed.
The nozzle plate is conventionally formed of a resin material or a metal such as stainless steel, nickel, aluminum and chromium. The resin material may be selected from among polyethylene terephthalate, polyimide, polyether imide, polyether ketone, polyether ether ketone, polyether sulfone and polycarbonate.
However, the method of producing the nozzle plate by nickel electrocasting as disclosed in JP-A-3-297651 suffers from a high cost of manufacture, and is not suitable for mass production of the nozzle plate.
On the other hand, the sheet-like nozzle plate 31 whose ink-jet holes 32 are formed by an excimer laser beam as shown in FIG. 18 tends to cause entry of air into the ink chamber 12, due to an insufficient volume of the ink-jet holes 32. The air remaining in the ink chamber 12 prevents smooth jetting of the ink through the ink-yet holes 32, resulting in deterioration of the quality of an image formed by the ink droplets. While the volume of the ink-jet holes 32 can be increased by increasing the thickness of the nozzle plate 31, an increase in the thickness causes difficult formation of the ink-jet holes 32 by the excimer laser beam. Further, an increase in the length of the ink-jet holes 32 results in an increase in the required voltage applied to the electrodes to deliver the ink droplets from the ink-jet holes 32.
The method of producing the nozzle plate by injection molding or forming the ink-jet holes by pressing or drilling also suffers from a problem. That is, the nozzle plate tends to have burrs around the edge of the ink-jet holes on its outer surface, for example, and the direction of jetting of the ink droplets from the ink-jet holes tends to fluctuate, leading to deterioration of the quality of the formed image. Although the injection molding method permits the ink-jet holes to have a sufficiently large volume, burrs 75, 85 are inevitably left at the outer or inner end of the orifice portion 72, 82 as shown in FIGS. 19 and 21. An experiment conducted on the nozzle plates 71, 81 of FIGS. 19 and 21 showed considerable fluctuation of the direction of ink jetting from the orifice portion 72, 82, namely, poor ink jetting stability.
According to the method in which the tapered ink-jet holes are formed by an excimer laser beam by changing the angle of the excimer laser beam path relative to the nozzle plate blank as disclosed in EP-A-0309146, the ink-jet holes can be formed with a sufficiently large volume due to their tapered shape. However, this method requires a long time to form the ink-jet holes, and suffers from low efficiency of mass production of the nozzle plate.
JP-A-63-31758 discloses an integrally formed piezoelectric member which has both ink chambers and ink-jet holes. Each ink-jet hole consists of an orifice portion from which an ink is delivered, and a tapered portion which communicates with the orifice portion and the ink chamber. The tapered portion has a diameter which decreases in a direction from one end adjacent to the ink chamber and the other end adjacent to the orifice portion. The orifice portion has a constant diameter over its entire length between the tapered portion and the outer surface of the nozzle plate. JP-A-63-31758 discloses the use of a laser beam to form the orifice portion.
If the ink-jet holes are formed by first forming tapered blind holes and then removing the bottom wall of the blind holes by a laser beam so as to form the orifice portion communicating with the tapered portion as indicated above, the nozzle plate will not have burrs which would be left around the edge of the orifice portion where the tapered and orifice portions of each ink-jet hole are simultaneously formed by injection molding. The nozzle plate thus formed assures high ink jetting stability.
However, it is difficult to form an integral piezoelectric member which has ink chambers and tapered portions of the ink-jet holes communicating with the ink chambers. In particular, the technique disclosed in JP-A-63-31758 is not applicable to the ink jet head of the type shown in EP-A-0277703, EP-A-0278589 and EP-A-0278590.